The published literature and patents describe a number of methods for preparing polypyrrole. The first polymerization of pyrrole was reported in 1916 to give a black amorphous powder long referred to as "pyrrole black". In recent years, polypyrrole and its derivatives have received increasing attention as an important class of electrically conducting polymers for their potential commercial and military applications in advanced materials, for example, electroactive, optical, anti-corrosion, electromagnetic-shielding and biomedical materials, and in construction of new types of other electronic materials, for example, microelectrodes, batteries, sensors and drug-release and electronic devices. Using appropriate processing methods, such as stretching of the polymer films or fibers or intercalative polymerization of pyrrole in an inorganic matrix, the conductivities of polypyrroles were reported to be as high as 500-7500 Siemens per centimeter (S/cm).
Polypyrrole and its derivatives can be prepared by (i) chemical or (ii) electrochemical oxidation of pyrrole and its derivatives as represented by the following equation: ##STR2##
The electrochemical polymerization of pyrrole is generally carried out using potentiostat, galvanostat or cyclic potential sweeping techniques in either aqueous or organic media. To obtain a fast rate of polymerization of pyrrole, a high applied potential of at least about 1.0 volts (V) versus (vs.) saturated calomel electrode (SCE) is generally required. However, the polypyrrole polymers so prepared decompose and/or undergo undesirable side-reactions readily at high potentials, often resulting in substandard quality of the polymers, including irreversible loss of electroactivity. Moreover, the processes disclosed in the scientific literature and patent literature for electrochemically polymerizing pyrrole generally provide reactions having slow reaction rates.
Chemical preparation of polypyrrole and its derivatives is usually carried out via oxidation of the monomers with oxidants, such as hydrogen peroxide and ferric chloride in both the liquid and vapor phases. Chemically prepared polypyrroles are usually of poor quality and possess lower conductivities than those prepared electrochemically. The yield of synthesis of chemically prepared polypyrroles is usually low unless a large amount of oxidant is used, which may cause many structural defects in the polymers.
Most of the previous work in the field of polypyrroles has been devoted to the study of the physicochemical and electrical properties of the polymers and to the study of new monomers. However, the chemistry of the polymerization of pyrrole and its derivatives has received little attention. Moreover, none of the previous methods for the synthesis of electrically conducting polypyrrole and its derivatives involves using organic initiators as in the present invention.
Accordingly, higher yielding and more effective cost and energy efficient methods of producing polypyrrole and its derivatives are needed.